1. Field of the Invention
This invention is related to a method of interference fringe analysis for determining aspects of the geometry of a subject surface, such as surface roughness and/or surface irregularities or unevenness, based on interference fringes generated between the subject surface and a reference surface.
2. Description of Related Art
In recent years, with a highly improved surface finishing accuracy of mechanical parts, optical parts and semiconductor parts, techniques are needed for measuring or examining the roughness of finished surfaces and/or the unevenness of finished surfaces with a high accuracy.
One such measuring or examining technique which is well known in the art is to analyze interference fringes or an interference pattern produced between two surfaces, namely a reference surface and a test subject surface, for examining surface conditions. Fizeau interferometers are also well known in the art and widely used to produce interference fringes on which an analysis is made. In order to produce contrasty interference fringes, it is essential for such a Fizeau interferometer that light is reflected in substantially the same quantity from the reference surface and the test surface. Because of the demand for quantitative consistency of reflected light between the two surfaces, when measuring or examining a test surface with a high reflectance, the reference surface must have a reflectance which is as high as that of the test surface. In this instance, what is called "multiple interference" is induced or led between the two surfaces, resulting in thin interference fringes. Another technique also well known in the art, is fringe scanning, in which interference fringe analysis is conducted by examining changes in brightness of interference fringes induced following changes in relative distance between a reference surface and a test surface. Because of a high measuring or testing accuracy, this fringe scanning technique is widely used for interference fringe analysis. Since the fringe scanning technique must provide interference fringes having a cos.sup.2 .THETA. brightness distribution, it is hardly available to highly reflective surfaces which generally produce thin interference fringes only.
When analyzing surface conditions from such thin interference fringes, the surface analysis is made based on pitches at which center lines of the respective thin interference fringes, which can be detected in conventional manners, are distributed and/or the roundness, or otherwise the straightness, of the respective thin interference fringes. For accurately measuring or examining a test surface, at least several interference fringes must be formed within a test area. In order to measure or examine an optically smooth test surface, the test surface is intentionally inclined relative to a reference surface so as to increase the number of interference fringes. However, if the test surface is inclined at a large angle relative to the reference surface so as to increase the number of interference fringes, since, in spite of a greatly increased amount of apparent information based on the geometric distribution of interference fringes, each interference fringe becomes nearly straight, it is hard to increase the substantial accuracy of measurement or examination. For this reason, the inclination of test surface relative to a reference surface is limited to some extent.
In the application of the conventional interference fringe analysis to automatic analyses, since the resolution of interference fringes depends upon the spatial resolving power of an image input device, such as TV cameras, it is also hard to increase the substantial accuracy of measurement or examination.
As described above, the number of points relative to the center of interference fringes, information or data of geometry relating to which are directly sampled, is inevitably determined based on the number of interference fringes, and the geometry of a test surface between adjacent interference fringes must be determined by means of interpolation. This leads to a greatly increased number of interference fringes desirably and precisely distributed over the test surface for accurate measurement or examination. Interpolation is available only to continuously and smoothly curved optical surfaces, such as polished surfaces which are ideally curved optical surfaces. Interpolation is, however, hardly available to metal surfaces finished by means of super mirror grinding. Moreover, interpolation is difficult or almost impossible to be applied to part of a test surface where no adjacent interference fringes appear. For this reason, it is desired to increase the number of interference fringes as much as possible so as to distribute them over the whole area of the test surface.